The present invention relates to exploration seismic surveying, and more particularly, relates to remotely detecting motions of the earth that are detected through the use of electromagnetic waves.
It is generally the objective of seismic exploration to generate seismic energy, make measurements of and record the amplitude of any reflected and refracted energy at selected locations and for selected times, and then by selectively processing the recorded seismic data, to deduce the geometry of the subsurface geologic boundaries as well as some of the properties of the materials of the earth through which the seismic energy has propagated and from which it has been reflected.
Conventional land seismic acquisition techniques involve the use of an appropriate source (dynamite, vibrator(s), airguns(s), etc.) to generate seismic energy and a set of receivers, spread out on the surface of the earth, to detect any seismic signals due to seismic energy interacting with subsurface geologic boundaries. These detected signals are recorded as a function of time and subsequent processing of these signals, i.e., seismic "traces" or seismic data, is designed to reconstruct an appropriate image of the geologic boundaries of the subsurface and to obtain information about the subsurface materials. In simplistic terms, this conventional acquisition process has a seismic wave, from a source of seismic energy, travelling down into the earth, reflecting from a particular geologic interface (i.e. a change or contrast in elastic constants, velocities, and/or densities), and returning to the surface, where the seismic wave may be detected by an appropriate receiver, or receivers.
Conventionally, the receiver employed to detect seismic signals on land is a geophone. A geophone is an electro-mechanical device that is coupled to the ground via an extension or "spike" that is physically inserted into the ground. This allows the geophone case to vibrate as a result of any earth motions, including seismic signals. Internal to the geophone case and vibrationally isolated from the case (typically by springs) is an "inertial" mass that does not vibrate with the earth. Thus, there is a small relative motion between the geophone case and its inertial mass due to any detected ground motions. This relative motion is converted to an electrical signal by having a coil of wires move through an electromagnetic field from a permanent magnet; the magnet may be the inertial mass with the coil attached to the geophone case, or vice versa. This electrical signal is the seismic signal that is recorded and later processed.
In general, seismic field layouts vary with the exploration objective sought to be detected. However, there is almost always a need to simultaneously record seismic motion at many ground positions spaced over a wide area. Conventional 2-D acquisition is usually limited to shot ("seismic energy source") locations and receiver locations with maximum spacings therebetween ("offsets") of about 10,000 meters (10,000 m) along a given survey line. A seismic line usually consists of multiple detector stations with each detector station made up of several detectors. The detectors are grouped together and connected so as to reject unwanted waves while enhancing the reception of desired seismic waves. The distance between stations is usually 25 to 50 m to allow for adequate spatial resolution of the subsurface geologic boundaries.
The signals from these arrays of detector stations are collected and recorded for each seismic shot (seismic energy source). A wavelet emitted for a seismic shot is usually a pulse of about 30 milliseconds (ms). Depending upon the depth of the layer of the subsurface from which they are reflected, received wavelets have lengths of 60-250 ms. The frequency spectrum of a received wavelet is typically limited at high frequencies, since the earth's absorption increases with frequency. The lower end of the frequency spectrum of a received wavelet is usually determined by detector sensitivity. Geophones have a natural vibration frequency at about 5-8 Hz but are damped to avoid any natural resonance at this frequency; this damping usually precludes acquiring seismic data below about 10 Hz. Thus, the normal operational seismic range is typically about 10-80 Hz. A typical geophone sensitivity is about 28 volts/(meter/sec).
The signal strength of the received wavelets measured with a geophone decreases with time after each shot until the signal reaches noise level, after which the wavelets cannot be identified anymore. The initial reflected signal level is typically a few hundred millivolts, while noise level is usually a few tens of microvolts (these voltages are rms values); the signal-to-noise ratio (SNR) runs from about 70 db (initial reflected signal) to 0 db (detection limit), with anything greater than about 30 db representing good signal quality.
In a conventional hypothetical land 3-D survey, shown in FIG. 1, each E-W line spans 6000 m with 60 m spacing intervals between each station (or array center) location 10 along an E-W line. Each array station may comprise, for example, 10-12 geophones. Four parallel lines, offset by 360 m N-S, are used simultaneously in this hypothetical example. This requires 400 active recording channels. After acquiring seismic data for several days, the bottom two E-W lines are each leapfrogged 1440 m to the North and the process is repeated until the detector spread has progressed the desired distance, i.e. covered the area of interest.
Such conventional land seismic data acquisition is slow, expensive and labor intensive. In particular, the activities of installing and then later removing the receivers or geophones is slow and labor intensive. The rapid expansion of 3-D acquisition, where many more source locations and many more receiver locations are used, puts even greater emphasis on developing more cost effective methods for receiving seismic signals.
It would be desirable to have many more channels so that finer spatial sampling and longer offsets in the N-S direction could be obtained during the activation of one seismic source, which may be, for example dynamite, vibrators, or air guns. It would also be desirable to have a fast way to move the entire detector spread or portions thereof and thereby minimize survey time.
Remote sensing of seismic vibrations is one potential solution for reducing the cost and time for acquiring seismic data. Remote sensing could be performed from an airborne platform (see FIG. 5) at some height above the earth area sought to be seismically surveyed.
However, conditions on the surface of the earth for remotely detecting seismic vibrations are quite different from most other remote detection conditions. More particularly, the minimum particle velocities associated with the seismic vibration amplitudes to be measured are very small (10.sup.-6 m/sec) and the natural reflectivity of the earth's surface is highly variable and is low in many cases. In addition to the desired seismic vibrations, the earth's surface also undergoes other types of vibrations. Further, the power of any radiation directed from an airborne platform to the ground must be limited in order to prevent eye damage to animals or people who look up at the radiation source. In addition, the height of the detecting platform is high to ensure adequate coverage of a large detection (survey) area. Finally, any detection scheme should also have a linear response over a wide range of seismic amplitudes.
U.S. Pat. No. 4,834,111 to Khanna et al discloses a heterodyne interferometer for measuring low vibration amplitudes of objects having low reflectivities; this heterodyne interferometer is principally for measuring inner or middle ear vibrations. Further, remote sensing vibration measurement equipment is commercially available. However, such equipment has a limited detection distance range (order of tens of meters) and use a very small target area (usually determined by a focused beam and having an approximate diameter of 50 to 100 micrometers). Such vibration measurement equipment is usually based upon laser Doppler interferometry.
U.S. Pat. No. 4,284,350 to Coon et al discloses a laser geophone. This reference employs a homodyne technique and as such is unable to distinguish an upward ground motion from a downward motion. Further, this reference employs no frequency shifting techniques and thus incurs much higher noise levels. The present invention is an improvement over the Coon, et al invention.
Rugged and transportable laser Doppler interferometer equipment have been developed for measuring wind velocities (wind shear) in front of aircraft and adjacent to airport runways. However, wind velocities are several orders of magnitude larger than seismic velocities (meters/sec versus .mu.meters/sec). These wind measuring systems have detection distance ranges of up to hundreds of meters and also use focused beams.
However, such known equipment is not well suited for remote seismic sensing. The single, focused, small spot is very sensitive to intensity variations from beam interruption and deflection, as well as local variations in spot reflectivity because of small beam movements at large distances. These intensity variations may cause a loss of signal.
These and other limitations and disadvantages of the prior art are overcome by the present invention, however, and new, improved methods and apparatus are provided for acquiring seismic signals with remote detection techniques which allow for faster seismic surveys.